1. Field of the Invention
This invention relates generally to a communications system incorporating an interleaver and, more particularly, to a digital communications coding system incorporating a permuted interleaver.
2. Discussion of the Related Art
Heavy demands are required on many different types of digital communication channels. Many times in these communication channels, a relatively large amount of bit errors may occur in a sequence of transmitted bits in a relatively short period of time. Errors occurring in this manner are generally referred to as error bursts, and hence, the channel is referred to as a bursty channel. Communication channels which may include error bursts of this type include scintillating channels, jam channels, ionosphere communications channels, or high interference channels. Additionally, the causes of error bursts can vary from one terminal type to another terminal type.
In bursty communications channels interleaving of the data bits is generally incorporated in order to make the communications more reliable. As is known in the art, interleaving is a process by which bit errors are decorrelated and separated from one another such that they can be more efficiently isolated and processed in an encoding and decoding communications process. The longer the duration of error bursts in a channel, the longer interleaving is required. However, longer interleaving requires additional information delay. Longer information delay degrades the quality of real time signals such as two way voice communications. An interleaver which separates errors the most for a fixed delay can greatly improve communications in bursty channels.
Conventional interleavers typically fall into three categories. These categories include block, convolutional and hybrid interleavers. Convolutional interleaving is currently the most widely accepted data interleaving scheme although the same concepts apply to all interleaving techniques. A typical convolutional interleaver design will guarantee that any encoded burst of errors of a particular burst length (BL) less than a predetermined number of bits (L) will be spread out so that these errors are at least a certain number of bits (D) apart when they reach the decoder input. The parameters D and L are dependent on the design of the interleaver. Bit error rate (BER) performance generally gets better as D and L get larger. However, this is at the cost of increasing communications delay. If a burst of errors lasts longer than the L number of bits supported by a particular interleaver design, then the errors will arrive at the decoder closer together than the number D bits normally maintained as a minimum separation by the interleaver.
The parameter D is generally determined by the specifics of the coding being used and by the desired level of BER performance. The parameter L is then determined by the interleaver designer. The choice of the parameter L is primarily driven by the expected channel burst length statistics in the allowable communications delay. Therefore, the total size and communications delay of the interleaver is proportional to (D.times.L). Therefore, if D must be large and the communications delay must be minimized in order to provide both reliable and fast communications, L must also be minimized. Since it is not always possible to make L larger than the longest expected burst of errors, performance of the interleaver for burst lengths greater than L will affect BER performance.
One problem attributable to the standard convolutional interleaver is that the separation of errors at the encoder input does not decrease slowly or steadily whenever the length of an error burst exceeds the interleaver value of L. For example, suppose that a particular interleaver design sets D=D.sub.1 and L=L.sub.1. If a burst of errors last longer than L.sub.1 bits, the errors will not be separated by at least D.sub.1 bits when they reach the decoder input. In fact, some of the errors will be separated by only one bit. In other words, the minimum separation of burst errors reached in the decoder input degrades from D.sub.1 to 1 as soon as the burst length exceeds L.sub.1.
What is needed then, is an interleaver design which not only provides the necessary separation of bit errors in a bursty communication channels during times when the burst of channel errors exceeds a predetermined number of bits, but in which the integrity of the interleaving process is substantially maintained during times when the bit error rate exceeds this number of bits. It is therefore an object of the present invention to provide such an interleaver.